1. Field of the Invention
The invention relates generally to seal assemblies for sealing between a rotating and a static member. In one aspect, and more particularly, the invention relates to seals for rolling cone bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. Still more particularly, the invention relates to multi-part dynamic metal seals that are employed to seal and protect the bearing surfaces between the rolling cone cutters and the journal shafts on which they rotate.
2. Description of the Related Art
An earth-boring drill bit is typically mounted on the lower end of a drill string. With weight applied to the drill string, the drill string is rotated such that the bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone.
A typical earth-boring bit includes one or more rotatable cone cutters. The cone cutters roll and slide upon the bottom of the borehole as the drillstring and bit are rotated, the cone cutters thereby engaging and disintegrating the formation material in their path. The rotatable cone cutters may be described as generally conical in shape and are therefore referred to as rolling cones.
Rolling cone bits typically include a bit body with a plurality of journal segment legs. The rolling cones are mounted on bearing pin shafts (also called journal shafts or pins) that extend downwardly and inwardly from the journal segment legs. As the bit is rotated, each cone cutter is caused to rotate on its respective journal shaft as the cone contacts the bottom of the borehole. The borehole is formed as the action of the cone cutters removes chips of formation material (“cuttings” or “drilled solids”) which are carried upward and out of the borehole by the flow of drilling fluid which is pumped downwardly through the drill pipe and out of the bit. Liquid drilling fluid is normally used for oil and gas well drilling, whereas compressed air is generally used as the drilling fluid in mining operations.
Seals are provided in glands formed between the rolling cones and their journal shafts to prevent lubricant from escaping from around the bearing surfaces and to prevent the cutting-laden, abrasive drilling fluid from entering between the cone and the shaft and damaging the bearing surfaces. When cuttings and/or abrasives are conveyed into the seal gland, they tend to adhere to the gland and/or seal component surfaces, and may cause deformation, damage and/or slippage of the seal components. Moreover, the cuttings can accelerate abrasive wear of all seal components and of the bearing surfaces.
In oil and gas drilling, the cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed before reaching the targeted formation. This is the case because each time the drill bit wears out or fails as a borehole is being drilled, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section by section in order to replace the bit. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. The amount of time required to make a round trip for replacing a bit is essentially lost from drilling operations. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. It is therefore advantageous to maximize the service life of a drill bit. Accordingly, it is always desirable to employ drill bits that will be durable enough to drill for a substantial period of time with acceptable rate of penetration (ROP).
The durability of a bit and the length of time that a drill bit may be employed before it must be changed depend upon numerous factors. Importantly, the seals must function for substantial periods under extremely harsh downhole conditions. The type and effectiveness of the seals greatly impact bit life and thus, are critical to the success of a particular bit design.
One cause of bit failure arises from the severe wear or damage that may occur to the bearings on which the cone cutters are mounted. These bearings can be friction bearings (also referred to as journal bearings) or roller type bearings, and are typically subjected to high drilling loads, high hydrostatic pressures, and high temperatures.
As previously mentioned, the bearing surfaces in typical bits are lubricated, and the lubricant is retained within the bit by the seals. The seal is typically in the form of a ring and includes a dynamic seal surface that is placed in rotating contact against a non-rotating seal surface, and a static seal surface that engages a surface that is stationary with respect to the seal ring. Although the bit will experience severe and changing loading, as well as a wide range of different temperature and pressure conditions, the dynamic and static seal surfaces must nevertheless remain sealingly engaged in order to prevent the lubricant from escaping and/or cuttings from entering the lubricated areas, and should perform these duties throughout the life of the bit's cutting structure.
A variety of seal types are known in the art. These include O-ring type seals made of rubber or other elastomeric material. The service life of bits equipped with such elastomeric seals is generally limited by the ability of the seal material to withstand the different temperature and pressure conditions at each dynamic and static seal surface.
Certain metal-to-metal seals have been employed in rolling cone bits. Such metal-to-metal seals were developed in order to increase the working life of the bearings, given that the failure of conventional elastomeric O-rings was one of the most frequent causes of bit failure. However, with metal-to-metal seals, great care and attention must be employed in their design, manufacture and assembly to ensure that, in use, the engaging sealing surfaces remain undamaged and in close contact with one another so as to ensure a good seal. In use, the bit will experience severe and varying loads, as well as a wide range of different temperatures and pressures. Under such a working environment, the cone cutters will tend to experience axial movement along the journal shaft, as well as radial movement, wobbling or rocking about the journal shaft. Such wobbling or rocking movement arises from the clearances inherent between the cone cutter and the journal shaft, and the extreme forces that are imparted to the cone cutter as it rotates about the borehole.
Excessive axial movement and/or wobbling of the cone cutter have the potential for damaging the seal components. More particularly, such components typically include relatively hard metal rings with very precisely machined and planar surfaces that must remain in good condition in order to seal effectively. However, the extreme and violent forces imparted to the cone cutter are, in turn, transmitted to these seal components. These forces may cause the seal elements to impact one another, thereby causing damage and lessening the life or effectiveness of the seal. The rocking or rolling motion of the cone cutter transmitted to the seal rings may likewise cause the sealing surfaces to become worn in a non-uniform way. Again, such damage or deformation is to be minimized. Where such seal components experience damage, the lubricant is able to escape, and cutting-laden drilling fluid is allowed to enter the seal gland causing still further deterioration and damage to the seal components. Eventually, enough cuttings may pass into the journal gap and enough lubricant may be lost such that rotation of the cone cutter is impeded and drilling dynamics are changed, eventually requiring the bit to be removed from the borehole. Accordingly, protecting the integrity of the seal is of utmost importance.
It is therefore desirable that a new, durable and long lasting seal assembly be devised, one having the benefits offered by metal-to-metal seals, including long life and relative insensitivity to high temperatures and pressures, but one that is not as susceptible to damage caused by impact loading transmitted from the cone cutter to the seal components.
Accordingly, to provide a drill bit with better performance and longer life, and thus to lower the drilling costs incurred in the recovery of oil and other valuable resources, it would be desirable to provide a seal that has the potential to provide longer life than conventional elastomeric seals and metal-to-metal seals. Preferably, such seals would provide a bit that will drill with acceptable ROP for longer periods so as to increase bit life and increase in footage drilled as compared to bits employing conventional seals.